Jim writes:
> But Wirt,
>
> Natural selection doesn't "cull" but rather it "favors." And selecting
> "for" something is very different than selecting "against" something.
> Favoring a trait leads to adaptation. That is, those with a trait leave
> more descendents. Even so, it is not that simple. At any rate, John
> Endler does a wonderful job of clearing things up with "Natural
> Selection in the Wild" and I highly recommend it for anyone who has not
> read it, and, don't forget, "The Extended Phenotype" by Dawkins, that
> should also be required reading.
Let me try one more time, if you don't mind. To do that, let me begin at the
beginning and write evolution as an algorithm. Given self-reproduction,
Darwinian evolution is composed of only these five components:
o a bounded arena
o a replicating population which must eventually expand beyond the
bounds of the arena
o thermodynamically inescapable replicative error, guaranteeing
variation within the reproducing population
o competition for space in that arena among the inevitable variants
o the consequential competitive exclusion of the lesser fit
If resources are more abundant than the population's current demands, then,
as a first order approximation, there is no effective competition among the
population's members, thus there is no selection. The competitive exclusion of
the least fit only begins in earnest when the resource space fills.
These few statements are the essence of Darwinian evolutionary ecology.
THE NATURE OF SELECTION
Let me apologize for stating the obvious, but given the comments on the list,
I thought it best to be as clear as possible. However, the simplicity of the
evolutionary algorithm doesn't mean that it can't be substantially
misinterpreted. In that regard, Malcolm asked a pertinent question:
> Am I understanding you correctly?
> Natural Selection selects against unfavorable phenotypes.
> Sexual Selection selects for favorable phenotypes.
No, unfortunately, you are misunderstanding me. The selective processes of
sexual and natural selection are similar, but the agents of selection are quite
different. They are similar however in that they both act to cull the least
appropriate individuals from the population.
Natural selection can be said to be the consequence of all of the extrinsic
forces that impinge on a population, but sexual selection is quite different,
startlingly so if you think about it for a minute. It is a mechanism of
selection that was invented within the phyletic lineage. It is a form of
selection
which the lineage imposes on itself, and it can be quite intense.
Two forms of error bedevil populations. They can be described as:
o design error
o manufacturing error
NATURAL SELECTION CORRECTS DESIGN ERRORS
Design error is associated with the population not being currently centered
on a local optimum, and thus rendering a population not as competitive as it
might be. Design error is quickly mitigated however by selection inexorably
moving the population across an apaptive topography to that point of maximum
optimality that is achievable in the current situation.
This movement is accomplished by constantly culling the least appropriate
("fit") of the excess population (that inevitable fraction of the population
above the carrying capacity of the current arena). At every stage of this
evolutionary movement, fitness is a relative quality. Some phenotypic trials
will be
more competitive than others, and their stochastic survival is more likely than
their less-competitive conspecifics.
I earlier gave the examples of longer tarsal hairs in barn flies and a 32
base-pair deletion in an allele of the CCR5 chemokine receptor in humans as
simple, point mutation instances that allowed populations to move extremely
rapidly
in the face of a drastically changed environment. Because these examples are
so simple, it seems perfectly reasonable to say that selection is selecting
"for" these properties, but this is not the normal condition, nor does it
present an accurate representation of the evolutionary process.
The interaction of a lineage's underlying genetic code with its manufactured
phenotypes is an extraordinarily complex process, being both highly polygenic
and pleiotropic. Because of this, selection cannot select for any single
quality in isolation of the remainder of the genotype. Rather, evolutionary
movement across an adaptive topography, as exemplified by the onset of either
endemism or the full speciation of a population as the lineage partitions a new
niche
for itself, must involve a "genotypic revolution," precisely as Ernst Mayr
argued.
SEXUAL SELECTION MITIGATES MANUFACTURING ERROR
The second form of error is "manufacturing" error. Although "design" error
can be effectively quelled by natural selection moving a population to a new
point of optimality, manufacturing error is inevitable and persistent, even
when
optimality has been achieved. This form of error is the inevitable result of
indefinitely replicating information in a positively entropic universe. It
cannot be suppressed other than by the phyletic lineages evolving/inventing
more
reliable information replication/transmission mechanisms.
The rate of infusion of novel mutational error into a germline is not small,
and if uncorrected would soon senesce the germline into noncompetitiveness or
inviability.
You can demonstrate this effect to yourself using a xerox machine. Find the
best xerox machine you can and make a copy of some detailed text. Then make a
copy of that copy, and repeat the process. Although the first copy may seem
perfect, by the 30th generation, the text will be unreadable. Exactly the same
problem faces the replication of the germline. If life on this planet were to
last more than a few tens of generations, solutions must have been devised.
Indeed this is probably one of the most pressing problems facing the evolution
of
living systems.
There are several alternatives that are afforded a phyletic lineage in order
to overcome this inevitable senescence. They are:
o produce a very large number of progeny at every generation, hoping
that some will be replicated without error. This solution comes at a horrendous
cost in efficiency. It also severely limits the complexity of the evolving
genome, and can be adopted only by the simplest of organisms.
o informational redundancy (high-order polyploidy) is an attractive
alternative until you calculate that the number of redundant copies must grow
to
infinity faster than the number of generations. Worse yet, redundancy becomes
an evolutionary trap, so stabilizing the germline that it becomes
non-evolvable, thus we find it to be very rare in nature.
o the evolution of error detection and repair algorithms provides for
mechanisms to slow down the rate of error infusion into the germline, and they
are effective. The evolution of these processes occurred early in the history
of life on this planet and are now pervasive, but they only slow the rate of
error infusion. They do not stop it. Although the evolution of mitotic error
detection and repair mechanisms much increases the complexity limit that's
allowed in the evolving genome, even under these conditions, it still remains
relatively shallow.
o perhaps surprisingly, the most effective means of excluding newly
introduced error from the germline are prolonged vigor demonstrations of one or
both genders, where the demonstrably least fit are excluded from the breeding
population.
WHY SEX?
Sexual reproduction offers great advantages to the evolving lineage, and
Alexey Kondrashov has cataloged 27 published explanations for the persistence
of
sex in the literature, but the most obvious remain these three:
o sex accelerates the evolution of a phyletic lineage to a point of
local optimality by sorting through the lineage's library of known working
variants, in stark contrast to waiting on random mutational events. This is the
traditional explanation.
o sex promotes the opportunity for a lineage to become a rapidly moving
target, thus mitigating "lockon" and exploitation by parasites. This
explanation is favored by Hamilton & Zuk.
o sexual meiosis offers the lineage the capacity to escape the aging
implicit in the apomixis associated with mitosis, making each generation new
again by recombining the working parts of two broken-down parents. This is the
explanation offered by Bell and others.
It is important to note however valuable each of these advantages are to a
sexual lineage, they do not require the evolution of gender differentiated male
and female castes. All of these advantages could be accrued by a single,
monomorphic, obligately outbred sexual form. That virtually no species on the
planet have adopted such a strategy has to be taken as first-order evidence
that
the evolution of sexual castes has significant advantages beyond those
attributed to the evolution of sexuality itself.
The advantages attendant to sexual selection are not equivalent to those of
sex itself.
SELECTION CULLS THE LEAST COMPETITIVE
The question whether selection selects "for" or "against" a quality is even
more clear in sexual selection than it is in natural selection. Jim recommends
that I read Dawkins, but I credit Dawkins for much of the confusion that
currently exists in evolutionary thought.
Dawkins believed, at least early on, in a nonsensical idea called "selfish
genetics," where one part of the genome would optimize itself at the expense of
the remainder of the germline. In the theoretical universe that he and others
built around this idea, he wrote that "we must expect lies and deceit" in the
process of males trying to maximize their access to females, and thus
maximizing their genetic representation in the next generation.
The opposite argument, "honest advertisement," is as starkly different from
Dawkins' selfish genetics as night is from day, and we have a mountain of
evidence that the evolutionary physics of honest advertisement is indeed in
effect,
beyond the theoretical considerations that no information-bearing system can
operate as Dawkins postulated.
Once anisogamy evolves, the phyletic lineage is offered a platform on which
it can greatly enhance those physiological differences with exaggerated
behaviors. These differences appear to be honed to very accurately segregate
defectively manufactured individuals from the breeding population before their
genetic
defects are reintroduced into the germline at the next generational juncture.
Males, in the vast majority of the complex Metazoa, appear to be a relatively
sacrificial gender, constructed to expose, exaggerate and expurgate novel
error from the germline at a significantly reduced cost as compared to
requiring
both genders to undergo the prolonged vigor demonstrations characteristic of
males.
If this is the primary purpose of a male, then how would you go about
designing such a sexual caste? The obvious answers are that you would want to
make
the caste as physiologically fragile as possible in order to more readily
expose
error, and you would make the caste pugnacious, ready to engage in combat at
the drop of a hat.
A great deal of evidence exists that males are a relatively physiologically
fragile, auxiliary sexual caste. Males, in the majority of animal species, live
shorter lives, are more heavily parasitized, less hygenic, are more likely to
succumb to starvation, trauma and stress, are less well genetically buffered,
are inherently more strategically sacrificial than females in times of
populational stress, and are often actively discriminated against during such
periods. Under these conditions, the prolonged and elaborate demonstrations of
competitive vigor and pugnacity characteristic of males act as an especially
effective genetic filter. This "honest advertisement" of vigor insures that
congenital defects present in the current population are not persistently
reintroduced
into the germline.
In a universe obeying these physical constraints, rather than evolving
polyploidy, the pressure lies in the opposite direction, to evolve haploid
structures. Diploidy is the lowest redundancy count that will allow for sexual
interchange while producing haploid gametes.
In the absence of sex, there would seem to be a design alternative that
seemingly would be successful: a haploid lineage that reproduces
parthenogenetically. Haploid individuals bearing signficant novel defects would
not reproduce
under such circumstances (no homologous allele exists to cover a defect in
these
circumstances), thus the phyletic lineage would be virtually absent of
genetic defects.
But we see no evidence for this design in nature, although there are a very
close alternatives: haplodiploidy, and its closely related condition,
parahaploidy.
Under haplodiploidy, males are haploid and thus made exceptionally fragile.
Most genetic defects will kill the embryonic males early on, but when a
substantial vigor demonstration is also added, the haploid male gender becomes
an
exceptionally intense filter of defects.
In the aculeate ("stinging") social hymenoptera, the common pattern is for a
newly emerged, winged diploid female to launch herself on a high-altitude,
high speed, long duration course, followed by a large number of haploid male
suitors. One or several of those males who are still with her at the end of her
course father the next generation. In this form of mating system, the bee
equivalent of a genetic defect such as cystic fibrosis can be guaranteed never
to be
introduced into the next generation. A mildly metabolically defective male
would never be competitive enough to stay with the group or be a potential
inseminator of the next generation.
In the United States every March we hold an equivalent of this contest in
collegiate basketball. The best sixty-four collegiate teams play a sudden death
playoff until the "number one" team is determined. It's obvious however that
the winner of this contest is highly random, given that so many of the games
are
decided by one point in the last few seconds of a game. What then does it
prove? Although we are psychologically hard-wired to celebrate the winner, the
contest is not fundamentally about winning.
If we were to run the NCAA contest one thousand times, using precisely the
same teams over and over again, exactly as they were on the first day, what we
would find is that some few number of the teams would win the vast majority of
the games. Beyond those few teams, there would also be a second-tier of teams
that won a one or two games. But there would also be a bottom tier of teams
that never won a single game out of the thousand, simply because they are
defective in some fundamental manner.
If basketball teams reproduced as organisms do, it wouldn't matter which of
the winningiest teams fathered the next generation, no more than it matters
which individual in the ball of males that surround the female at the end of
her
flight inseminates her. Sexual selection is not about determining "winners."
It is a process designed to maximally assure the elimination of the "losers,"
the genetically defective.
It's very important to understand this difference. If sexual selection
operated as Dawkins' proposed that it did, where "we should expect lies and
deceit,"
not only would the germline very rapidly senesce due to relicative entropy,
it would be even more radically corrupted by the female selecting "for" some
relatively narrow trait that could be easily counterfeited. But if the purpose
of the contest were to exaggerately expose and expurgate error from the
germline by eliminating its defect bearers from the germline, the phyletic
lineage
becomes, after a fashion, immortal.
Wirt Atmar
